We didn't make the first body change until almost 2 years in, along with the decision to commit fully, and the intent to be exclusive life partners (not marriage or other "entrapment", just the express purpose of being each other's dream come to life). No permanent action was taken until we had grown to love and trust each other intimately and like family, but the communication concerning my fetish happened in the first few weeks of dating.
Part of success in this is finding a person who has the potential to be open to your fetish, but it is my opinion that it's much better if she at least understands your general fetish during your dating process. Probably not on the first date, but the sooner the better. If someone is digging you and yet the mere description of your fantasies scares her off permanently, she isn't cut out to go down this road with you. She doesn't have to share your desires or even committed to them for your sake, but she should understand you. If you wait months or years to drop bombs like sexual preferences you are not only asking for explosive emotional trauma, but establishing a dishonest approach that only makes things worse.
I attribute our success to being open and honest from the beginning. I knew she wasn't a gold digger by her personality and also because I wasn't spoiling her and blowing money on her, but I'm sure it helps that I didn't meet her by advertising that I was looking to make a plastic doll either.